The present invention relates generally to current sensing devices for electrical systems, and more particularly to fault indicators for alternating current power systems that are capable of automatically selecting one of a plurality of trip settings.
Various types of self-powered fault indicators have been constructed for detecting electrical faults in power distribution systems, including clamp-on type fault indicators, which clamp directly over cables in the systems and derive their operating power from inductive and/or capacitive coupling to the monitored conductor; and test point type fault indicators, which are mounted over test points on cables or associated connectors of the systems and derive their operating power from capacitive coupling to the monitored conductor.
Some prior art fault indicators detect a fault by monitoring the rate of change of current in the monitored conductor over time, such as by using the current derivative, di/dt. For example, the rate of change of current may have to exceed 100 amperes (A) within 50 milliseconds (ms). However, if a fault occurs with a slower current derivative, the fault may go undetected and no fault indication will occur.
Other prior art fault indicators may be either of the manually resetting type, wherein it is necessary that the indicators be physically reset, or of the self-resetting type, wherein the indicators are reset upon restoration of line current. Examples of such fault indicators are found in products manufactured by E. O. Schweitzer Manufacturing Company of Mundelein, Ill., and in U.S. Pat. Nos. 3,676,740, 3,906,477, 4,063,171, 4,234,847, 4,375,617, 4,438,403, 4,456,873, 4,458,198, 4,495,489, 4,974,329, 5,677,678, 6,016,105, 6,133,723 and 6,133,724.
Detection of fault currents in a monitored conductor by a prior art fault indicator is typically accomplished by magnetic switch means, such as by a configured magnetic reed switch, in close proximity to the conductor being monitored. Upon occurrence of an abnormally high fault-associated magnetic field around the conductor, the magnetic switch actuates a trip circuit that produces current flow in a trip winding to position an indicator flag visible from the exterior of the indicator to a trip or fault indicating position. Upon restoration of current in the conductor, a reset circuit is actuated to produce current flow in a reset winding to reposition the target indicator to a reset or non-fault indicating position, or the fault indicator may be manually reset.
Rather than a target indicator, some prior art fault indicators utilize light emitting diodes (LEDs) to display a fault condition. Whether of the target or LED display type, prior art fault indicators typically have a pre-set or pre-programmed trip setting, such as 100 A, 200 A, 400 A, 800 A, 1200 A, or the like. This means that a variety of fault indicators must be manufactured, inventoried and sold to satisfy each specific load current application.
There is therefore a need for a fault indicator with multiple trip settings that can determine the present load current level in a monitored conductor and select one of the trip settings that is appropriate for present load current conditions on the monitored conductor.
There is a further need for such a fault indicator that is normally in an energy conservation mode and that periodically wakes up to monitor the load conditions on the monitored conductor, or is awakened when a fault condition occurs, when the fault indicator is reset after a timed reset period, or when a microcontroller in the fault detector otherwise receives an input or interrupt signal.
In certain other applications, the need arises for a fault indicator that will continue to display a prior fault condition for a predetermined amount of time, such as in the range of one hour to twenty-four hours, rather than self-resetting upon restoration of current in the conductor. Such timed reset fault indicators should be capable of self-resetting after termination of the predetermined time.
Some of these applications also require voltage in-rush restraint and/or current in-rush restraint to prevent false tripping due to voltage and/or current inrush, such as when a reclosing relay of a power distribution system closes.
Because of the compact construction and limited power available in self-powered fault indicators, it is preferable that the desired functions of the fault indicator be accomplished with minimal structure and with internal circuitry that has minimal current drain on a high capacity battery. The fault indicator must also provide highly reliable and extended operation over a number of years.
Accordingly, it is a general object of the present invention to provide a new and improved fault indicator that automatically selects one of a plurality of trip settings that is related to the load current in the monitored conductor.
Another object of the present invention is to provide a fault indicator that is capable of operating over a broad range of load currents, such as from about 5 A to about 2500 A.
Yet another object of the present invention is to provide a fault indicator with a plurality of trip settings that periodically reevaluates the load current and that automatically adjusts the trip setting to a higher or lower setting if the load current has changed to a different range than previously determined.
A further object of the present invention is to provide a fault indicator that initially displays a permanent fault upon the occurrence of a fault in the monitored conductor, that reevaluates the monitored conductor after about one minute, and that continues to display the permanent fault condition if the load current is then below about 5 A.
Another object of the present invention is to provide a fault indicator that initially displays a permanent fault upon the occurrence of a fault in the monitored conductor, that reevaluates the monitored conductor after about one minute, and that changes the display to a temporary fault condition if the load current is then greater than about 5 A.
A still further object of the present invention is to provide a fault indicator that initially displays a permanent fault when the load current rises above the present trip setting, that reevaluates the monitored conductor after about one minute, and that adjusts the trip setting to a high trip setting that is related to the higher load current.
Another object of the present invention is to provide a fault indicator that automatically determines whether it is in an overhead or underground application, and that automatically configures itself for the overhead or underground application.